Do Upper Wilcox Canyons Support Paleogene Isolation of the Gulf of Mexico?

摘要

Previously identified deeply incised valleys of the Upper Wilcox along the onshore mid-Texas Gulf Coast are renamed as non-generic canyons. The canyons have seven general characteristics. They incise two different stratigraphic horizons within the Upper Wilcox as defined by maximum flooding surfaces. Log cross sections and seismic data support their interpretation as incisions not lateral facies changes. They are predominately shale filled and extremely deep (up to 770 ft or 235 m near the shelf edge), cutting through the Yoakum maximum flooding surfaces below. They incise the shelf up to 12 mi (19 km) and are linked to slope canyons. On the slope they are in the company of slope-confined canyons. Shales of the canyons overtop the incisions. Finally, 3.1 mi (5 km) distant from the shelf edge, clinoforms fill the Anna Barre canyon. They initially downlap the base of the canyon, are primarily thick foresets (488 ft or 149 m) with dips of 7 degrees, similar to delta clinoforms. The have a rising clinoform trajectory that aggrades from 254 ft (77 m) to 98 ft (30 m) from the canyon top. The canyon fill facies include stacked shoreface sandstones, stacked canyon fill delta sandstone, channel sandstones, central basin (lagoonal) shales within the shelf portions of the canyon, and marine shales in the slope or outer portions of the canyons. The stacked shoreface sandstones are 240 ft (73 m) thick with the lowest sandstones further from the shelf edge than the uppermost sandstones. They are strike parallel, bar the canyon shelf entrance, and shale out where the canyon is deepest. The stacked canyon fill deltas consist of an areally small lower sequence and a more broadly deposited upper system. The base is 190 ft (58 m) from the top of the canyon. Channel sandstones have a meandering geometry, thicknesses up to 135 ft (41 m), and their base is some 310 ft (94 m) from the top of the canyon. Seismically they are one cycle thick with a flat top and bottom. High resistivity shales represent central basin fill (lagoonal) muds, whereas low resistivity shales are interpreted as marine shales. The canyon origin is proposed as a shelf incised submarine canyon. However, the canyon fill facies represent a relative sea level rise and transgression, implying a previous relative sea level fall within the canyon. Facies thicknesses indicate relative sea level changes in excess of those recently proposed for the Paleogene world ocean. The magnitude of isostatic rebound is not enough to explain this large relative change. Other explanations are needed, including consideration of recent proposals involving isolation of the Gulf of Mexico from the world ocean.